A shaft of a gas turbine engine is generally supported by a fluid bearing which are hydrodynamically lubricated. In particular, so called tilting pad bearings may be used for supporting the shaft.
A combined tilting pad bearing module has a three bearing surfaces, one is a rotor journal support for transferring radial forces and two are axial thrusting surfaces, one of which is designated an ‘active’ side (generally designed for receiving the largest thrust) intended for receiving the axial thrust at the operating point of the gas turbine, and the other is an ‘inactive’ side (generally designed for receiving a limited thrust) required for brief periods of transient off design conditions of the turbine.
Generally the design intent of bearings of this combined bearing module type with respect to ‘inactive’ and ‘active’ sides, is that the bearing is to be used ‘directionally’, where the ‘active’ side is arranged relative to a main thrusting direction of the rotor.
Due to instrumentation requirements for some gas turbines as either a result of legislation or customer requirement, which may include but not limited to vibration monitoring or axial displacement probes, a non-symmetrical design (in terms of number of thrust pads and e.g. lubrication nozzles) of bearing may be required to be used, and therefore it will not be possible to have a bearing module with equal thrust capacity on each side of the module, again, determining a bearing ‘directionality’.
Over the operational life of the gas turbine (in particular a turbine in a single shaft design), external effects such as temperature/humidity of environment (which affecting inlet conditions), gas turbine filtration/cleanliness or various states of component deterioration/fouling can cause the net thrust balance of the gas turbine to change. Minor changes in net thrust balances causes a variation in thrust loads imparted on these bearings, which would normally be within the capabilities of the bearing modules thrust load ratings and would be no cause for concern. However, significant changes in net thrust balance can cause the engine to operate in the opposite rotor thrust direction with respect to the design intent, where the ‘inactive’ side of the bearing module effectively becomes the main thrusting side at the operating point, which inherently has less thrust capacity, and has been known to cause failures, as the thrust loads exceeds the bearing module thrust load capacity rating on this side.
GB 309, 124 discloses radial-flow turbines which are arranged in two housings, so that the end-thrusts on the common shaft balance one another. The steam may flow through the radial-flow turbines in series or in parallel. A thrust collar is provided and a driven machine is arranged between the radial-flow turbines.
WO 2013/011150 A1 discloses a rotary machine with a rotor that is mounted in a bearing. The rotor is subjected to a thrust that acts substantially in only an axial direction during the operating time. The thrust is received and dissipated by a first thrust bearing of the bearing via sliding means. The bearing further comprises a second thrust bearing, which generates a force that acts on the first thrust bearing in the direction of the thrust at least temporarily at the same time as the thrust occurs.
EP 1 479 875 A1 discloses a bearing for an axial support of a gas turbine rotor. The bearing has a bearing component in which is installed a hydraulic piston arrangement with which communicates a hydraulic system. Restrictors and flow control valves are interposed between the hydraulic piston and hydraulic system and may be installed in the bearing component. The hydraulic piston arrangement has a number of pistons installed in respective piston chambers which are constructed as cylindrical bores.
U.S. Pat. No. 3,784,266 discloses a hydrodynamic pivoted segmental thrust bearing which comprises centrally supported, pivoted bearing segment including at least one elongated lubricating fluid collection groove disposed in a bearing surface of the segment adjacent and parallel to at least one of the radial edges thereof. The bearing segment further including at least one fluid outlet aperture disposed through the segment opening at one end into the groove and at the other end into the thrust bearing well of the bearing. The collection groove drains fluid from the bearing surface into a bearing well, and interrupts the formation of the bearing surface lubricating fluid film, thereby reducing an effective bearing surface on one side of the segment.
GB 2 453 143 A discloses a thrust bearing comprising a high melting-temperature thrust collar and thrust pad. Mutually facing surfaces of the collar and pad are coated with a coating of high hardness and low coefficient of friction. The substrate is preferably steel and the coating is preferably a ceramic coating, more preferably a carbon-enriched tungsten carbide coating. The invention is chiefly applicable to the thrust bearing of a turbocharger.